Photographic materials in which silver halides are used are employed in a wide range of applications at the present time. In the field of photosensitive materials for printing purposes, a strong demand has arisen for faster printing and development processing operations. In fact, techniques have been developed over the years for shortening processing times and these techniques have been introduced commercially. In practical terms, these techniques have involved increasing the speed of the photosensitive materials (shortening the printing process), increasing the rate of development (shortening the development processing time) and increasing durability, with respect to scratching etc. which can occur as a result of increasing line speeds. Furthermore, stability in development processing is of importance in addition to the points indicated above.
Methods in which the amount of light absorbed per silver halide grain is increased and methods in which the efficiency of latent image formation, with respect to the quantity of light absorbed, is increased can be considered for increasing the speed of a silver halide emulsion.
It is thought that in the former case, the amount of light which is absorbed per grain can be increased by increasing the size of the silver halide grains or, in cases where the emulsion is spectrally sensitized, it is thought that the amount of light absorbed can be increased by increasing the amount of spectrally sensitizing dye. However, it is known that the rate of development often decreases when the size of the silver halide grains is increased. Furthermore, it is known that increasing the amount of spectrally sensitizing dye inhibits development or de-silvering. Hence, in most cases it is difficult to employ these methods in practice.
In the latter case, the independent or combined use of methods of sulfur sensitization, gold sensitization or reductive sensitization or other so-called chemical sensitization methods, is effective. However, these is a limit to the speeds which can be achieved using these methods. That is to say, in many cases, increased fogging and a lowering of contrast occur when short exposures are made at high brightness levels, due to excessive sensitization.
Hence it is important that the emulsion speed should be raised while holding the size of the silver halide grains constant and without invoking the adverse effects mentioned above.
It is known that the rate of development of silver halide emulsions can be increased not only by reducing the size of the grains in the emulsion, but also by using silver chlorobromide which is essentially free of silver iodide and which, moreover, has a high silver chloride content. However, in most cases it is difficult to obtain a high speed with silver chlorobromide which is essentially free of silver iodide and which has a high silver chloride content. Hence, the raising of the emulsion speed is also of importance from the point of view of increasing the rate of development.
As mentioned earlier, there is a great need for providing silver halide photographic materials with an effective increase in speed without adversely affecting their high speed development properties. Furthermore, it is anticipated that while achieving higher speeds, the materials will also have excellent stability in processing and resistance to pressure in handling.
A method of forming emulsion grains by the socalled halogen conversion method has been disclosed in Japanese Patent Publication No. 36978/75 corresponding to U.S. Pat. No. 3,622,318 as an example of a method for obtaining high speed silver halide emulsions. However, although emulsions prepared using this method exhibit increased speed, it has been found that the speed is markedly reduced when pressure is applied to the photosensitive material. It has been discovered that this can be minimized by reducing the amount of halogen conversion, but in this case, fogging is liable to occur when pressure is applied to the photosensitive material and the gradation obtained is softer.
Furthermore, a number of techniques concerning the so-called laminated type emulsion in which the silver halide grains have layers of different halogen composition within them have been reported as methods of providing both superior development rates and high speeds.
For example, emulsions prepared by covering a silver bromide core with silver chloride or precipitating a layer of silver bromide on a core of silver chloride have been disclosed in Japanese Patent Publication No. 8939/81 corresponding to GB Patent 1,027,146 as a means of realizing the advantages of both of these materials. However, the technique disclosed therein is a wide ranging technique covering, in general, laminated type emulsions in which the grains consist of a core of silver halide which is covered with a layer of a different silver halide. According to experimental results obtained by the present inventors, it is not always possible to obtain emulsions which have the preferred performance in this way. For example, with emulsions prepared using the above mentioned technique, reversal images are liable to form in regions which have received a comparatively low level of exposure in many cases and there is a further disadvantage in that considerable desensitization occurs when pressure is applied to the emulsion. Moreover, in many cases the gradation obtained is soft and there are also cases in which the toe of the characteristic curve has a soft gradation and there are two levels of gradation.
Furthermore, a technique in which a laminated type silver chloroiodobromide emulsion which has a silver bromide content of at least 50 mol% in the outermost layer is chemically sensitized with an unstable sulfur compound in the presence of fine silver chloride grains has been disclosed in Japanese Patent Application (OPI) No. 9137/83. However, when attempts were made to prepare laminated type emulsions using the technique disclosed therein, the gradation of the toe of the characteristic curve was liable to be softened and it was also found that the material was desensitized when pressure was applied.
Moreover, it has been reported that techniques involving laminated type structures are effective for raising the speed of silver chlorobromide emulsion which have a high silver chloride content.
For example, techniques concerned with high silver chloride emulsions which have laminated type structures have been disclosed in Japanese Patent Application (OPI) Nos. 95736/83 and 108533/83 corresponding to U.S. Pat. No. 4,564,591. According to the former, high speed emulsions which can be processed quickly can be obtained by providing a layer consisting principally of silver bromide within the grains. However, when experiments were carried out in practice, it was found that desensitization was liable to occur when pressure was applied to the emulsion grains and such materials would be difficult to use in practice. Furthermore, with the latter technique, emulsions which can be processed quickly and which have a high speed and which, moreover, have a wide latitude for chemical sensitization are obtained by providing a layer consisting essentially of silver bromide on the surface of the grains. However, when experiments were carried out it was found that here again there were disadvantages; i.e., the toe of the characteristic curve was liable to be soft and, in extreme cases, two levels of gradation were observed, and furthermore the material was liable to desensitization by pressure.
Techniques involving high silver chloride emulsions which have a laminated type structure have also been disclosed in Japanese Patent Application (OPI) Nos. 222844/85 corresponding to U.S. Pat. No. 4,590,155 and 222845/85 corresponding to U.S. Pat. No. 4,605,610. However, the disadvantages of the type described above have not been overcome by theses techniques.